Niobium-Catalyzed Intramolecular Addition of O-H and N-H Bonds to Alkenes: A Tool for Hydrofunctionalization

Article abstract of DOI:10.1021/acs.orglett.7b00657

A convenient, versatile, and easy to handle intramolecular hydrofunctionalization of alkenes (C-O and C-N bonds formation) is reported using a novel niobium-based catalytic system. This atom economic and eco-friendly methodology provides an additional synthetic tool for the straightforward formation of valuable building blocks enabling molecular complexity. Various pyran, furan, pyrrolidine, piperidine, lactone, and lactam derivatives as well as spirocyclic compounds are produced in high yields and selectivities.

Full text of DOI:10.1021/acs.orglett.7b00657

Letter
pubs.acs.org/OrgLett
Niobium-Catalyzed Intramolecular Addition of O−H and N−H Bonds
to Alkenes: A Tool for Hydrofunctionalization
Laura Ferrand, Yue Tang, Corinne Aubert, Louis Fensterbank, Virginie Mouries-Mansuy, Marc Petit,*
̀
and Muriel Amatore*
́ ́
Sorbonne Universite, UPMC Univ Paris 06, Institut Parisien de Chimie Moleculaire, UMR CNRS 8232, Case 229, 4 Place Jussieu,
75252 Paris Cedex 05, France
S
* Supporting Information
ABSTRACT: A convenient, versatile, and easy to handle
intramolecular hydrofunctionalization of alkenes (C−O and
C−N bonds formation) is reported using a novel niobium-based
catalytic system. This atom economic and eco-friendly method-
ology provides an additional synthetic tool for the straightfor-
ward formation of valuable building blocks enabling molecular
complexity. Various pyran, furan, pyrrolidine, piperidine,
lactone, and lactam derivatives as well as spirocyclic compounds
are produced in high yields and selectivities.
Scheme 1. Eco-Friendly Hydrofunctionalization of Alkenes
evelopment of environmentally friendly and economical
Dprocesses is absolutely critical for the construction of
valuable synthetic targets. Over the past decades, catalysis has
proven to be an effective strategy to rapidly access molecular
complexity with consideration of step/atom economy, energy
and resources saving, or waste minimization. While associated
with limited availabilities, expensive costs, and identified toxicity,
noble metals catalysis has been nevertheless utilized in most of
the commonly used methodologies. With this in mind, the
exploitation of inexpensive, nontoxic, and abundant low-valent
transition metals in catalysis is an ongoing field of research.
Inspired by Obora’s work,1 we recently identified NbCl3·DME as
a competent catalyst for an unprecedented [2 + 2 + 2]
cycloaddition to access fully substituted benzosilacyclobutenes.2
Despite its valuable benefits in terms of sustainability, toxicity,
and cost, niobium chemistry is still in its infancy and most of the
reported literature is dedicated to the formation of coordination
complexes and materials mainly for industrial applications.3
Because of their unusual reactivity compared to other transition
metal halides, niobium complexes such as NbCl5 or NbCl3·DME
have received particular attention either as a Lewis acid or as a
precursor of reactive Nb(III)-alkyne or -imine complexes, and
some niobium-mediated processes have been developed.2,4
With the goal of broadening the toolbox for synthetic chemists,
we set out to more deeply explore the reactivity of NbCl5 as a
catalyst for the intramolecular hydrofunctionalization of
unactivated alkenes (C−O and C−N bonds formation, Scheme
1).5 Such completely atom economic transformations promoted
by either various and often precious transition metals (Au, Pt, Ru,
Re, Zr, Ti, Al, Bi, Mg, ...)6 or Brønsted acids7 have been deeply
studied showing contrasting results. Unfortunately, despite their
great potential, only few transition-metal-based catalytic systems
have shown suitable versatility regarding substrates and the direct
use of strong organic acids often demonstrates inferior catalytic
performance due to their inherent instability and difficulty to
handle. Indeed, to the best of our knowledge, processes with a
broad scope, low cost, and low environmental impact are limited
to the use of Ag,8 Ln,9 Cu,10 Co,10a,11 Fe,10a,12 and Ca13 catalysis
(Scheme 1). However, none of these metals exhibits generality in
each of the following cyclizations: unsaturated alcohols, amines,
carboxylic acids, and amides,14 moreover the success of the
reactions often requires high catalyst loadings (up to 25 mol %),
an excess of additives (silver salts up to 30 mol %, bases,
disiloxanes, oxidants, ammonium or pyridinium salts), ionic
liquids, zeolithes, and sophisticated ligands. In this context,
niobium catalysis emerges as a novel eco-friendly alternative for
the construction of molecular complexity. Indeed this metal
offers low toxicity, is affordable, and is relatively abundant in the
earth’s crust.3,15
Received: March 3, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00657
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We first selected unsaturated alcohol 1a as a hydroalkoxylation
model substrate. Comparison of the cyclization efficiency using
catalytic amounts of niobium(III) or (V) catalysts clearly
demonstrated the supremacy of the cationic NbCl5/AgNTf2
system at room temperature (Table 1, entries 1−7; see
loading could be reduced to 1 mol % of niobium at the cost of a
prolonged reaction time (Table 1, entry 15).
After optimization of our catalytic system, we investigated its
performance in terms of substrate scope. The direct access to
substituted tetrahydrofurans and tetrahydropyrans was first
envisaged (Table 2). The cyclization of unsaturated alcohols
Table 1. Catalyst Screening and Optimization of the Reaction
Conditions
Table 2. Scope of Unsaturated Alcohols
conversion
(2a + 3a/1a)
yield
(%)
a
a
entry
cat. (mol %)
(2a/3a)
b
1
2
3
4
NbCl3·DME (10)
NbCl5 (10)
(0/100)
(0/100)
(0/100)
(13/87)
nr
−
−
−
(91/9)
nr
nr
10
Nb(OEt)5 (10)
NbCl3·DME/AgClO4
(10)
5
6
NbCl5/AgClO4 (10)
NbCl5/AgNTf2 (10)
NbCl5/AgSbF6 (10)
NbCl5/AgNTf2 (10)
(58/42)
(54/46)
(21/79)
(71/29)
(87/13)
41
51
12
62
75
(88/12)
(95/5)
(100/0)
(96/4)
(94/6)
c
7
d
8
d
9
NbCl5 (10)/AgNTf2
(20)
de
,
10
11
12
NbCl5 (10)/AgNTf2
(20)
(100/0)
(92/8)
69
63
70
(90/10)
(93/7)
(94/6)
df
,
NbCl5 (2.5)/AgNTf2
(5)
dg
,
NbCl5 (2.5)/AgNTf2
(5)
(90/10)
a
The ratio 2a/3a (96/4) was determined by 1H NMR analysis.
Reaction was carried out at 50 °C. Reaction was carried out at 80 °C.
b
c
dg
,
13
14
15
NbCl5 (2.5)
AgNTf2 (5)
(0/100)
(0/100)
(90/10)
nr
nr
74
−
−
(94/6)
dg
,
dh
,
1a−h proceeded smoothly at 50 or 80 °C in 6 h (Table 2, entries
1−8). The features of the process appeared closely related to the
substitution pattern of the double bond, and a highly
regioselective cyclization generally occurred at the most
substituted carbon. To our great delight, quantitative yields of
tetrahydrofurans 2b and 2c were obtained (Table 2, entries 2 and
3). The substitution on the tether had only an influence on the
reaction rate, and unsubstituted alcohols 1f and 1g were cyclized
with satisfactory yields up to 97% at 80 °C (Table 2, entries 6 and
7). Finally, the terminal 2- allylphenol 1h could afford the
corresponding cyclized compound 2h with a 66% yield and
complete regioselectivity (Table 2, entry 8).
NbCl5 (1)/AgNTf2
(2)
a
The conversion (2a + 3a/1a) and ratio (2a/3a) were determined by
b
c
1H NMR analysis. No reaction was observed. Due to the modest
conversion observed using AgSbF6, AgNTf2 was retained as the
additive of choice. However, additional studies were realized to further
evaluate AgSbF6 efficiency on this reaction as a cheaper alternative
d
(see Supporting Information, part 4). The reaction was conducted in
e
dichloroethane (DCE). Reaction was conducted at 80 °C for 10 min.
f
g
The reaction was conducted at 80 °C for 30 min. Reaction was
h
conducted in DCE for 6 h at 50 °C. Reaction was performed on 1 g
scale in DCE for 24 h at 50 °C.
The reactivity of alkynes was also tested in this niobium-
catalyzed hydroalkoxylation reaction. Interestingly, spirocycliza-
tion of diol 4 proceeded efficiently and yielded the valuable spiro-
(5,6) compound 5 as the sole product (Scheme 2).
Supporting Information, parts 2 and 3 for more detailed
optimization studies).16 From screening various types of solvent,
chlorinated solvents were found to give the best efficiency with
dichloroethane (DCE) as the best candidate (Table 1, entries 6
and 8). Further tuning of thestoichiometry of the NbCl5/AgNTf2
catalytic system was realized, and it was found that using a 1:2
ratio delivered a mixture of cyclized compounds 2a and 3a in 75%
yield with high selectivity (Table 1, entries 8 and 9). Increasing
the reaction temperature to 80 °C led to a full conversion of 1a,
and in that case catalyst loadings could be decreased with only a
slight loss of yield (Table 1, entries 10 and 11). Finally, 1a
underwent smooth hydroalkoxylation at 50 °C affording a
mixture of 2a and 3a after 6 h in good yield (70%) and selectivity
(94/6) (Table 1, entry 12). In contrast, conducting the reaction
in the absence of NbCl5 or AgNTf2 was not productive,
emphasizing the beneficial effect of the catalytic combination
NbCl5/AgNTf2 (Table 1, entries 13 and 14). Interestingly, this
one-step process was readily gram-scalable and the catalyst
Scheme 2. Niobium-Catalyzed Spiro-Compound Synthesis
To explore the full scope of this reaction, we investigated the
ability of unsaturated amines to undergo cyclization (Table 3).
Unfortunately, no hydroamination occurred with unprotected
primary amine 6a presumably due to reaction inhibition through
strong but nonproductive coordination to niobium17 preventing
any activation of the amine moiety or through nondesired
trapping of a proton source that could be in situ delivered (Table
B
DOI: 10.1021/acs.orglett.7b00657
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Encouraged by these results we further explored the versatility
of our method by conducting hydrocarboxylation and hydro-
amidation reactions (Table 4). The optimized conditions could
Table 3. Scope of Unsaturated Amines
Table 4. Scope of Unsaturated Carboxylic Acids and Amides
a
b
c
No reaction was observed. Full conversion was observed. Product
8h′ arising from undesired hydroarylation.
3, entry 1; see SI, part 6 for preliminary mechanistic
investigations). Identically, most of the N-protecting groups
proved unsuccessful (6b−d, Bn, Bz, Boc, Table 3, entry 2).
Gratifyingly, this niobium catalytic system could catalyze the
desired cyclization with nucleophiles such as sulfonamides 6e−i
(Table 3, entries 3−7). While the corresponding pyrrolidines
7e−g (Table 3, entries 3−5) and piperidine 8i (Table 3, entry 7)
could be isolated with high selectivities and excellent yields up to
99%, the reaction of sulfonamide 6h featuring a prenyl moiety
gave product 8h in low yield with isolation of an undesired
compound8h′ as themajor product arising from ahydroarylation
reaction (Table 3, entry 6).
be applied to a variety of unsaturated carboxylic acids and amides
yielding the corresponding lactones (Table 4, entries 1−3) and
lactams (Table 4, entries 4 and 5) thus illustrating the synthetic
potential of our methodology. Surprisingly and in marked
contrast to previous results, the cyclization of the carboxylic acid
homologue of 1a led to low selectivity (Table 4, entry 1).
However, high regioselectivities and satisfactory yields were
achieved by modifying the substitution pattern of the double
bond (Table 4, entries 2 and 3) or switching to amide-type
analogs (Table 4, entries 4 and 5).
In summary, we have established an atom economic method
for the intramolecular hydrofunctionalization of alkenes with
high yields and selectivities. Being an efficient strategy for the
straightforward synthesis of a large panel of heterocyclic
compounds as valuable scaffolds for natural products and
pharmaceuticals, this niobium-catalyzed reaction provides a
nice and attractive contribution to the synthetic toolbox. While
our given methodology, centered on a relatively inexpensive
niobium-based catalyst, is convenient, versatile, and easy to use,
the exact nature of the niobium active species and the reaction
mechanism are still to be determined and are subject to further
investigations (see SI, part 6 for preliminary mechanistic
investigations). As initial results, control experiments using
PhSiMe3 as an efficient noncoordinative proton scavenger have
suggested a mechanism involving a direct interaction between the
metallic species and the substrate in contrast to a hidden
Brønsted acid catalytic pathway. Gaining better insight into the
reaction mechanism is necessary to explore the full potential of
this method and to develop more attractive asymmetric versions.
We next examined the outcome of the reaction of unsaturated
amino-alcohols under our catalytic conditions (Scheme 3). Once
Scheme 3. Hydroalkoxylation versus Hydroamination
a
b
No reaction was observed. Compound 13 was obtained as an
inseparable mixture of four diastereoisomers and ratios could not be
attributed.
again, reaction was totally suppressed when using nonprotected
amino-alcohol 9a. As seen previously, hydroxy-sulfonamide 9b
was the best candidate and afforded heterocyclic compounds 10b
and 11b in good yields with hydroalkoxylation as the favored
pathway (see SI, part 5 for additional studies on reaction
chemioselectivity). Finally, the synthetic potential of our method
was highlighted by the possibility to readily form spirocyclic
compound 13 in 81% yield as a valuable scaffold.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00657.
Detailed experimental procedures and spectral data for all
products are provided (PDF)
C
DOI: 10.1021/acs.orglett.7b00657
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: marc.petit@upmc.fr.
*E-mail: muriel.amatore@upmc.fr.
ORCID
Muriel Amatore: 0000-0002-6455-5633
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.7b00657
Org. Lett. XXXX, XXX, XXX−XXX